1. Field of the Invention
The present invention relates to a DC-DC converter, and particularly relates to a DC-DC converter of which efficiency is improved.
2. Background Art
Two types of batteries, namely, a high-voltage battery and a low-voltage battery are usually provided in an electric-powered vehicle such as an EV (Electric Vehicle), an HEV (Hybrid Electric Vehicle), or a PHEV (Plug-in Hybrid Electric Vehicle).
The high-voltage battery is mainly used as a power supply for a load of a high voltage (hereinafter, referred to as a high-voltage load), such as a main power motor for driving wheels of the electric-powered vehicle to travel and a compressor motor of an air conditioner.
On the other hand, the low-voltage battery is mainly used as a power supply for a load of a low voltage (hereinafter, referred to as a low-voltage load), such as a motor for various ECUs (Electronic Control Units), EPSs (Electric Power Steerings), electric brakes, car audio devices, windshield wipers, and power windows, or an illumination lamp.
For example, a DC-DC converter steps down the voltage of the high-voltage battery to supply to the low-voltage battery in order to charge the low-voltage battery.
Conventionally, in a switching supply such as the DC-DC converter, various efforts have been made to reduce a power loss so as to enhance the efficiency.
For example, there is proposed a switching supply device that changes a frequency according to an output power so as to reduce a switching loss (see, for example, Japanese Unexamined Patent Publication No. 2004-222429).
There is also proposed a DC-DC converter which reduces a current amount necessary to charge or discharge a gate capacity of a switching transistor in a consumption current necessary to generate an internal clock by lowering a frequency in pulse-width modulation control of the switching transistor during a light load (see, for example, Japanese Unexamined Patent Publication No. 2000-201473).
There is proposed a power supply circuit in which the high energy conversion efficiency is achieved by varying an oscillation frequency of a DC-DC converter according to an output current of the DC-DC converter (see, for example, Japanese Unexamined Patent Publication No. 10-323027).
There is proposed a high-efficiency, high-performance stabilizing power supply device in which power saving is achieved by varying a switching frequency during a high load of a load device (see, for example, Japanese Unexamined Patent Publication No. 2004-328834).
There is proposed a switching supply device in which a frequency is changed from a low switching frequency to a high switching frequency at a changing current value I1 when an output current exists in a increasing process and the frequency is changed from the high switching frequency to the low switching frequency at a changing current value I2 lower than the changing current value I1 when an output current exists in a decreasing process, thereby achieving the high efficiency (see, for example, Japanese Unexamined Patent Publication No. 2007-68349).
Although not aimed at the high efficiency, in a power conversion control device that is used as both an inverter for driving a motor of an electric vehicle and a converter for charging a battery, in order to solve a noise problem, the number of switching times is decreased as less as possible to reduce a loss of the switching element because of a large current when the motor is driven, and a carrier frequency is set to a frequency in an ultrasound frequency range because of a small current when the battery is charged (see, for example, Japanese Unexamined Patent Publication No. 7-336812).
Generally, in a DC-DC converter, the efficiency is degraded during the low load, that is, the low power consumption of the load of the DC-DC converter. This is because a ratio of the power necessary for the control of the output power of the DC-DC converter increases relative to the output power of the DC-DC converter or a resonant power supply used in the DC-DC converter cannot perform ZVS (Zero Voltage Switching) during the low current.
On the other hand, when the electric-powered vehicle is connected to an outlet for household use for the purpose of the charging (so-called plug-in charging), the DC-DC converter is operated to change the low-voltage battery while the electric-powered vehicle is stopped. In this case, in the DC-DC converter, the efficiency is degraded because the output power is much smaller than that during the operation of the electric-powered vehicle. The sum of the power loss of the DC-DC converter increases because it takes a long period of time (for example, 8 hours) to perform the plug-in charging.